In recent years, a digital coherent optical communication technology capable of compensating for various types of distortion generated on a communication channel by a digital signal processing technology has been attracting attention in the optical communication field. Nonlinear distortion, a type of distortion generated on a communication channel, is caused by polarization not being proportional to an electric field of a light wave, mainly when intensity of a light wave is high. Nonlinear distortion includes self-phase modulation in which phase change is caused by intensity of the own light wave, cross-phase modulation in which phase change is caused by another light wave in wavelength division multiplexing or the like, four light wave mixing in which two or more light waves interact and generate a new light wave, and the like.
While a bit rate used in the optical communication currently under research and development exceeds 100 Gbps, a digital signal processor is able to operate only at a few GHz (Gbps: Gigabits per second, GHz: Gigahertz). In other words, the optical communication is approximately 100 times faster than digital signal processing. In order to fill the rate gap between the optical communication and the digital signal processing, the same number of digital signal processing circuits as the number of input signals arranged in parallel need to be arranged.
For example, even a process that can be performed by a single complex multiplier requires arrangement of 100 complex multipliers in order to fill the rate gap between the optical communication and the digital signal processing, and the circuit scale is several Mega gates. In an LSI that performs high-speed signal processing, a circuit scale of a single complex multiplier is several Mega gates and easily exceeds a design limit of an LSI (LSI: Large Scale Integration). Therefore, reduction in a circuit scale of an LSI performing high-speed signal processing is an important issue.
FIG. 11 illustrates an example of a general FIR filter 110 (FIR: Finite Impulse Response).
The FIR filter 110 in FIG. 11 includes a delay group 111, a multiplier group 113, and an adder group 115. The delay group 111 includes a plurality of delay (111-1, 111-2, 111-3, . . . , 111-N−1) (N: natural number equal to or greater than 2). The multiplier group 113 includes a plurality of multipliers (113-1, 113-2, . . . , 113-N−1, 113-N). The adder group 115 includes a plurality of adders (115-1, 115-2, . . . , 115-N−2, 115-N−1).
Since the FIR filter 110 illustrated in FIG. 11 requires one multiplier for implementing one tap, a digital filter needed in nonlinear compensation requires several hundred multipliers. Therefore, a circuit scale of a digital filter in a high-speed LSI such as an optical digital coherent transceiver is several hundred Mega gates.
FIG. 12 illustrates an example of a general IIR filter 120 (IIR: Infinite Impulse Response).
The IIR filter 120 in FIG. 12 includes a delay group 121, a first multiplier group 123, a second multiplier group 124, a first adder group 125, and a second adder group 126. The delay group 121 includes a plurality of delay (121-1, 121-2, . . . , 121-N). The first multiplier group 123 includes a plurality of multipliers (123-1, . . . , 123-N−1, 123-N). The second multiplier group 124 includes a plurality of multipliers (124-1, 124-2, . . . , 124-N, 124-N+1). The first adder group 125 includes a plurality of adders (125-1, 125-2, . . . , 125-N). The second adder group 126 includes a plurality of adders (126-1, 126-2, . . . , 126-N).
The IIR filter 120 illustrated in FIG. 12 can realize a filter having a better amplitude characteristic with a fewer number of multipliers than an FIR filter.
FIG. 13 is a configuration example of an IIR filter in which only a[1] has a nonzero value and zero otherwise, implemented on a system in which two signals are input in parallel. The IIR filter 130 in FIG. 13 includes adders 131 and 133, multipliers 132 and 134, and a delay device 135.
Further, as one of effective methods compensating for nonlinear distortion, there is a method called a back propagation method in which distortion generated on a communication channel is compensated in reverse order. The back propagation method includes a method compensating for only self-phase modulation as described in NPL 1 and a method compensating for not only self-phase modulation but also cross-phase modulation with another polarized wave as described in NPL 2.